On Polar Cap Dynamics under Strong Solar Wind Driving
Abstract
We investigate how polar cap dynamics, quantified by the polar cap (PC) index, respond to solar wind direct driving and magnetotail energy unloading during intervals of exceptionally large (10 mV/m) reconnection electric field. Using 53 one to two-day intervals that include such extreme fields, we find that, among 11 candidate coupling functions including the electric field of Kan and Lee (1979) and the universal coupling function of Newell et al. (2007), the PC index correlates most closely with the electric field of Kivelson and Ridley (2008), EK-R, a form in which the electric field imposed on the ionosphere by low-latitude magnetopause reconnection saturates at high levels of geomagnetic activity. It is found that nightside magnetospheric processes, as represented by an unloading AL index (ALU), make a significant contribution to the PC index. A linear model is constructed to relate the PC index to its solar wind driver and magnetotail driver, i.e. PC ≈ β0 + β1zs(EK-R) + β2zs(ALU), where β0, β1 and β2 are estimated from regression, and zs( ) is a normalization function. Based on this model, it is estimated that the portion of the PC index directly driven by the solar wind electric field outweighs the contribution arising from energy release in the magnetotail by roughly a factor of 2. The solar wind dynamic pressure (pdyn) does not play a key role in controlling the PC index. However, under intense solar wind driving, the number density (n) can influence the solar wind-magnetosphere coupling by changing the solar wind Alfvén conductance, which is incorporated in EK-R. The validity of the linear model is verified by comparing its results with those obtained from a more general, non-linear model, PC ≈ α + f1(EK-R) + f2(ALU), where α is a constant, and f1( ), f2( ) are arbitrary smooth functions. It is found that, except in anomalous events during which the auroral oval expanded poleward to the latitude of the PC index station and the index increased because of proximity to auroral zone currents, the linear model is a good approximation, since more than 70% of the variation in the PC index is explained by the model. The linear model provides a useful tool to study the coupling between the solar wind, magnetosphere and ionosphere.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMSM23A2299G
- Keywords:
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- 2712 MAGNETOSPHERIC PHYSICS / Electric fields;
- 2736 MAGNETOSPHERIC PHYSICS / Magnetosphere/ionosphere interactions;
- 2776 MAGNETOSPHERIC PHYSICS / Polar cap phenomena;
- 2784 MAGNETOSPHERIC PHYSICS / Solar wind/magnetosphere interactions